The present invention relates to pushbutton switches for use in electrical circuits and in particular to pushbutton switches which exhibit a stable on operating characteristic. The stable on characteristic prevents an intermittent change of state from occurring during a partial actuation of the switch.
Pushbutton switches are well known in the art and examples thereof may be found in U.S. Pat. Nos. 3,694,603, 4,055,736 and 4,345,128 the disclosures of which are incorporated herein by reference. The foregoing patents disclose pushbutton switches adapted to sequentially open and close an electrical circuit or to sequentially switch a common lead between two alternate portions of a circuit. In addition to the switches shown in the above patents, other designs are known in the art, having a variety of configurations, adapted for similar function.
A problem often found with pushbutton switches is an instability caused by a partial actuation of the switch's plunger. When in the closed state, a very small depression of the plunger causes the switch to momentarily open. Upon release of the plunger, the switch returns to its original state.
Pushbutton switches are often used in conjunction with cosmetic housings having a switch actuating lever or button. Such housings may be found on industrial machinery, consumer appliances and automotive vehicles. Various manufacturing and assembly problems are encountered in the mounting of a pushbutton switch in such a housing. For example, tolerance stacking may cause the housing lever to hold the switch plunger in a partially actuated position. External vibration may then cause the switch to intermittently open when it is in the closed state.
In an attempt to resolve this problem, the prior art has suggested the use of various actuating mechanisms to achieve a switch exhibiting a stable on characteristic. However, up to the present, the alternatives proposed have incorporated features introducing problems and disadvantages in their operation.
For example, the switch disclosed in U.S. Pat. No. 4,906,808 to Burgess et al., directly addressed the instability problem. In the older art referenced above, switch operation includes a contact member which is raised and lowered and which may additionally be rotated. The contact is rigidly fixed to a plunger and ratcheting mechanism. The contact's movement away from the terminals of the switch begins with the initial movement of the plunger.
In the '808 patent, the plunger and ratcheting mechanism are not rigidly fixed to the contact member. Contact movement is solely rotational and begins near the end of the plunger downstroke. This allows for a partial movement of the plunger with no effect on the state of the switch. However, to achieve this desired stable on feature, the '808 switch incorporated several disadvantages well known in the art. The switch utilizes contact bridging, actuation on the downstroke and terminal edge-contact impacting. These features result in a relatively high magnitude of contact wear, a relatively high plunger actuating force and increased manufacturing costs.
The contact bridging arrangement requires that the contact simultaneously engage two terminals to close the switch. The contact is biased into communication with the terminals by a spring. A minimum force is necessary between the contact and each terminal to insure good electrical communication. As there are two terminals, the required force of the spring is greater than that which would be required if the contact itself acted as a terminal and communication was required with only one other terminal, a non-bridging arrangement. The required greater spring force results in increased wear between the contact and the terminals and increases the force needed to actuate the switch.
As the switch operates on the downstroke, when the spring is in a nearly fully compressed state, the force of the spring is at its highest during switch actuation. This high spring force acts against the contact member at the time it is sliding over the terminals causing a relatively high rate of friction and wear between the contact and the terminals.
Finally, in operation, the contact member initially impacts the side and top connecting edge of each terminal. Then due to ramping forces developed by the shape of the contact, the contact moves up over this edge and finally comes to rest on the top of the terminals. This impact greatly increases the wear of the terminals and the contact. In order to minimize this wear, it is necessary to keep the height of the terminal edge, relative to the plane on which the contact member rotates, within very close tolerances. Maintaining these tolerances requires precision molding, increasing the cost of manufacture of the switch.
There is, therefore, a need for an improved pushbutton switch which provides a stable on feature with a normal actuating force, a low wear characteristic, and which is economical to manufacture.